1. Field of the Invention
The subject matter of this invention is related generally to electromagnetic contactors and more specifically to apparatus for sensing contactor current.
2. Description of the Prior Art
Electromagnetic contactors are well known in the art. A typical example may be found in U.S. Pat. No. 3,339,161 issued Aug. 29, 1967 to J. P. Conner et al. entitled "Electromagnetic Contactor" and assigned to the assignee of the present invention. Electromagnetic contactors are switch devices which are especially useful in motor starting, lighting, switching and similar applications. A motor starting contactor with an overload relay system is called a motor controller. Contactors and motor controllers sense the electrical current flowing through the closed contacts and use information related to that current for useful purposes. Often the current is sensed with classical well known current transformers where an output current which is generally proportional to the input current is provided. The traditional current transformer approach has a number of disadvantages. One disadvantages lies in the fact that traditional current transformers are only usable over a relatively small range of input current. To measure input currents outside of this range the current transformer must be replaced by a larger or smaller one. Another disadvantage lies in the fact that current transformers are typically relatively large and relatively heavy. This is very disadvantageous in situations where it is desirous to minimize the size of motor controller or contactor parts. It would be desirous therefore to provide a motor contactor or controller which utilized a current transformer or transducer which was relatively small and yet useful over a wide range of input current. Until now it has been difficult to find current transformer arrangements which meet both of the above-named criteria. For example, U.S. Pat. No. 4,454,557 issued June 12, 1984 to Hurley and entitled "Non-Linear Alternating Current Transducer" teaches a transformer device which is useful over a relatively wide range but which accomplishes the latter by deliberately providing a non-linear output. Furthermore, the transformer is utilized in various stages of saturation which is usually associated with a relatively heavy transformer with higher losses. Other current transducer arrangements which are of interest in view of the present invention are: U.S. Pat. No. 3,197,721 issued July 27, 1965 to S. E. Bogotch et al., entitled "Toroidal Apparatus with Windings Surrounding Both Core and Gap Wherein Permeable Material is Variably Positioned" and U.S. Pat. No. 4,007,414 issued Feb. 8, 1977 to Shima et al., entitled "Current Transducer Arrangement".
Many of the problems associated with the aforementioned art are solved by an invention disclosed in copending concurrently filed U.S. patent application Ser. No. 016,422 (W.E. Case 53,126) entitled "ELECTROMAGNETIC CONTACTOR WITH LIGHTWEIGHT WIDE RANGE CURRENT TRANSDUCER" by J. A. Bauer and assigned to the assignee of the present application. Even this improvement however presents a problem associated with the transducer output being influenced by the affect of stray magnetic flux. This problem has been solved in the past by utilizing a current transformer in which the secondary winding thereof is disposed about a non-magnetic insulating material in an even number of successive layers The inner layer extends from a first point on the core along substantially the entire core length to a second point closely adjacent the first point then the outer layer extends from the second point back to the first point. The winding turns in these layers are wound about the core in the same direction and both layers have a substantially equal number of turns distributed substantially uniformly along the length of the core. This has a tendency to reduce the affect of stray magnetic fields however it requires the use of a relatively tedious winding process which is time consuming and relatively expensive. Such a current transformer is taught in U.S. Pat. No. 3,449,703 issued June 10, 1969 to F. L. Steen and entitled "Current Transformer Having an Accuracy Unimpaired by Stray Flux From Its Adjacent Conductors". Iron powder toroidal cores which have relatively low permeability are disclosed in a publication entitled "Pyroferric" published by Pyroferric International Inc. of Toledo, Ill. It would be advantageous if a current transducer could be found which could be utilized with a microprocessor controlled motor contactor or controller which had the advantages associated with the previously described copending application and which was not impaired by stray magnetic flux but was nevertheless relatively simple and inexpensive to construct. U.S. Pat. No. 4,368,424 issued Jan. 11, 1983 to R. C. Miller and entitled "Mutual Inductance Current Transducer For AC Electrical Energy Meters" teaches the use of both the discrete gapped core and air core current transducers described previously with respect to the copending concurrently filed U.S. patent application Ser. No. 016,424 (W.E. Case 53,126). It would be advantageous if a new transducer could be found to replace the immediately described Miller transducer for electric energy meters and which could be accurately utilized over a very large range of input current to produce a useful output.